10/12/10. Chapter 16. A Macroscopic Description of Matter. Chapter 16. A Macroscopic Description of Matter. State Variables.

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1 Chapter 16. A Macroscopic Description of Matter Macroscopic systems are characterized as being either solid, liquid, or gas. These are called the phases of matter, and in this chapter we ll be interested in when and how a system changes from one phase to another. Chapter Goal: To learn the characteristics of macroscopic systems. Chapter 16. A Macroscopic Description of Matter Topics: Solids, Liquids, and Gases Temperature Phase Changes Ideal Gases Ideal-Gas Processes State Variables Parameters for macroscopic description of matter Volume, pressure, mass, mass density, thermal energy, moles, number density, temperature Density The ratio of a system s mass to its volume is called the mass density, or sometimes simply the density. The SI units of mass density are kg/m 3. In this chapter we ll use an uppercase M for the system mass and lowercase m for the mass of an atom. The mass of an atom is determined primarily by its most massive constituents, the protons and neutrons in its nucleus. The sum of the number of protons and neutrons is called the atomic mass number A. The atomic mass scale is established by defining the mass of 12 C to be exactly 12 u, where u is the symbol for the atomic mass unit. The conversion factor between atomic mass units and kilograms is 1

2 By definition, one mole of matter, be it solid, liquid, or gas, is the amount of substance containing as many basic particles as there are atoms in 12 g of 12 C. The number of basic particles per mole of substance is called Avogadro s number, N A = mol 1. The number of moles in a substance containing N basic particles is If the atomic mass is specified in kilograms, the number of atoms in a system of mass M can be found from I am pretty familiar with the concept of mole, and Avogadro s number. A. TRUE B. FALSE The molar mass of a substance is the mass in grams of 1 mol of substance. The molar mass, which we ll designate M mol, has units g/mol. The number of moles in a system of mass M consisting of atoms or molecules with molar mass M mol is Which system contains more atoms: 5 mol of helium (A = 4) or 1 mol of neon (A = 20)? QUESTION: EXAMPLE 16.2 Moles of oxygen A. They have the same number of atoms. B. Helium C. Neon 2

3 EXAMPLE 16.2 Moles of oxygen EXAMPLE 16.2 Moles of oxygen I am pretty familiar with the concept of temperature, different temperature scales, and their conversions. A. TRUE B. FALSE Temperature The Celsius temperature scale is defined by setting T C =0 for the freezing point of pure water, and T C =100 for the boiling point. The Kelvin temperature scale has the same unit size as Celsius, with the zero point at absolute zero. The conversion between the Celsius scale and the Kelvin scale is The Fahrenheit scale, still widely used in the United States, is defined by its relation to the Celsius scale, as follows: Temperature 3

4 Phase Changes The temperature at which a solid becomes a liquid or, if the thermal energy is reduced, a liquid becomes a solid is called the melting point or the freezing point. Melting and freezing are phase changes. The temperature at which a gas becomes a liquid or, if the thermal energy is increased, a liquid becomes a gas is called the condensation point or the boiling point. Condensing and boiling are phase changes. The phase change in which a solid becomes a gas is called sublimation. State of Matter Phet Simulation I have visited the Phet website and played with these simulations. A. TRUE B. FALSE Phase Changes Ideal Gases The ideal-gas model is one in which we model atoms in a gas as being hard spheres. Such hard spheres fly through space and occasionally interact by bouncing off each other in perfectly elastic collisions. Experiments show that the ideal-gas model is quite good for gases if two conditions are met: 1. The density is low (i.e., the atoms occupy a volume much smaller than that of the container), and 2. The temperature is well above the condensation point. 4

5 The Ideal-Gas Law The pressure p, the volume V, the number of moles n and the temperature T of an ideal gas are related by the idealgas law as follows: EXAMPLE 16.3 Calculating a gas pressure QUESTION: where R is the universal gas constant, R = 8.31 J/mol K. The ideal gas law may also be written as where N is the number of molecules in the gas rather than the number of moles n. The Boltzmann s constant is k B = J/K. EXAMPLE 16.3 Calculating a gas pressure EXAMPLE 16.3 Calculating a gas pressure Ideal-Gas Processes Many important gas processes take place in a container of constant, unchanging volume. A constant-volume process is called an isochoric process. Consider the gas in a closed, rigid container. Warming the gas with a flame will raise its pressure without changing its volume. Ideal-Gas Processes Other gas processes take place at a constant, unchanging pressure. A constant-pressure process is called an isobaric process. Consider a cylinder of gas with a tight-fitting piston of mass M that can slide up and down but seals the container so that no atoms enter or escape. In equilibrium, the gas pressure inside the cylinder is 5

6 Isobaric Process What is the ratio T f /T i for this process? A. B. C. 4 D. 2 E. 1 (no change) What is the ratio T f /T i for this process? Isothermal Process A. B. C. 4 D. 2 E. 1 (no change) Quasi-Static process A process that is essentially in thermal equilibrium at all times Above you see the isotherms for four different isothermal processes. The arrow points in the direction of A. Increasing temperature B. Decreasing temperature C. Neither Increasing nor decreasing 6

7 Applications 7